FIG. 1 illustrates a known type of system for driving and dimming LEDs. Many different types of systems are known.
A conventional DC-DC converter 12 receives an input voltage Vin. The converter 12 typically boosts the voltage to slightly above the combined forward voltages of the series connection of LEDs 14, such as 40 volts, in order to cause a predetermined regulated current to flow through the LEDs 14. Any type of converter may be used, such as a flyback converter. Other types of converters, such as buck converters, may be used depending on the relationship between the input voltage and the required driving voltage.
The converter 12 adjusts the duty cycle of the MOSFET 16 (or other type of switch) so as to supply the desired current through the LEDs. The duty cycle, for a current mode converter, is set by the converter regulating the peak switch current per switching cycle. The peak current is determined by the voltage across the resistor Rs.
The value of the resistor RL in series with the LEDs 14 may be selected to set the LED current level, when current flows through the LEDs 14, or a separate current control signal may be applied to an input terminal of the converter 12 to set the current level.
The converter 12 may instead be controlled to output a regulated output voltage Vout by applying a feedback voltage Vfb (a divided Vout) to an internal error amplifier, where the duty cycle of the MOSFET 16 is controlled to cause Vfb to match a reference voltage.
An output circuit 18 typically comprises a diode, and inductor, and a capacitor, where the configuration determines the type of converter 12, such as flyback. A synchronous rectifier may be used instead of a diode.
The converter 12 typically has various other features that prevent over-voltages and over-currents from occurring.
The converter 12 contains an oscillator that may have a fixed frequency or an adjustable frequency. In the example of FIG. 1, the frequency of the oscillator is set by an external resistor Rt value. A fixed current flows through the resistor Rt, and the resulting voltage Vt sets the oscillator frequency. Increasing the resistor Rt value increases Vt and the switching frequency.
Therefore, the converter 12 and its related components set a fixed current or fixed voltage for driving the LEDs 14. In order to control the dimming of the LEDs 14, an external PWM dimming controller 22 is supplied that controls the switching of a series MOSFET 24. An external control signal Vdim, which may be a voltage, a current, a resistance, or other variable signal, controls the duty cycle of the pulses output from the controller 22. The duty cycle controls the average current though the LEDs 14. The controller 22 typically has a fixed switching frequency, such as 100 Hz, that is high enough to avoid perceptible flicker by the LEDs 14 turning on and off. The percentage of time that the LEDs 14 are on determines the perceived brightness. The controller 22 operation is independent from the converter's switching of the MOSFET 16.
Due to the high switching frequencies of the converter's MOSFET 16, such as 100 KHz-5 MHz, and the resulting signals conducted by the inductor and capacitor in the output circuit 18, some electromagnetic interference (EMI) may be generated by the components. To spread the RF power, it is known to continuously vary the switching frequency over a range. This technique is referred to as spread spectrum frequency modulation (SSFM).
FIG. 2 illustrates how a controllable oscillator 26 in the converter 12 may be controlled with a ramping spread spectrum control (SSC) signal applied to an external pin to continuously vary the switching frequency of the MOSFET 16. In the example of the converter 12 of FIG. 1, the SSC signal may modulate the voltage Vt to cause the switching frequency to vary from the base frequency set by resistor Rt.
However, the present inventors have discovered that, due to the SSFM, noticeable LED flicker occurs. There is ringing and ripple in Vout caused by the converter's switching and the interaction of the inductor and capacitor in the output circuit 18. The phase of the ringing and ripple is directly related to the phase of the converter's internal oscillator. Additionally, the switching of the LED dimming MOSFET 24 causes a surge of current to flow through the LEDs 14, which briefly lowers Vout at the beginning of a PWM dimming pulse. When using SSFM, the characteristics of Vout are thus continually changing with the switching frequency and direction of the frequency ramp, including the speed at which the converter 14 corrects for a changing load current. For example, a slower switching frequency results in a slightly longer time for the peak current through the MOSFET 16 to be corrected for changes in load current. Additionally, the converter may go into a discontinuous mode as some slower frequencies but stay in a continuous mode at higher frequencies. Therefore, as the switching frequency changes, the speed that the converter 12 recovers from the surge of current changes, and the ripple and ringing change. For these various reasons, as the converter's 14 switching speed changes, the current waveform through the LEDs 14 near the beginning of each LED PWM pulse is different for each LED PWM pulse. This variation from pulse to pulse is perceptible to the observer as flicker, even though the LEDs 14 may be pulsed at about 100 Hz.
FIG. 3 illustrates the independence of the switching frequency of an SSFM converter (ramping between 300 KHz to 400 KHz) and the occurrence of the LED PWM pulses. In other words, the period of the spread spectrum control (SSC) ramp is different from the period of the LED PFM pulses. Note how the switching frequency is different at the start of each LED PWM pulse, such as 375 KHz, 325 KHz, etc. This causes perceptible flicker. In FIG. 3, it is assumed that a rising ramp increases the switching frequency, but in other types of systems a rising ramp decreases the switching frequency.
What is needed is a technique that allows for SSFM control of a switching regulator and PWM dimming control of LEDs without any resulting perceptible flicker.